Studies on fatigue life enhancement of pre-fatigued spring steel specimens using laser shock peening
Introduction
Fatigue is one of the primary reasons for the failure of structural components. The life of a fatigue crack has two parts, initiation and propagation. In high cycle fatigue (HCF), with a polished part and no stress raisers, about 90% of the life is spent in fracture initiation [1]. In addition to enhancing fatigue life of components through various techniques, fatigue life extension of partly fatigue damaged in-service components remains area of high commercial interest. Fatigue life extension is usually achieved either by extending crack initiation period (before the appearance of a microscopic crack) or retarding crack propagation (after appearance of microscopic crack). Surface treatment has been identified as an effective tool for both delaying crack initiation as well for fatigue crack retardation. Both the approaches rely on modifying surface residual stress field or microstructure. Compressive residual stresses are known to significantly increases fatigue life and fatigue strength by inhibiting initiation and propagation of cracks [2], [3]. Under fatigue loading conditions, compressive residual stress introduced in the surface layer by mechanical surface treatments (e.g. shot peening, autofretage, hole expansion, laser shock peening and low-plasticity burnishing [4]), experience cyclic relaxation [5]. Reintroduction of compressive stress field in partly fatigue-damaged specimens can set the clock back, thereby effecting extension of component’s residual fatigue life. A local heating method is a popular technique for extending fatigue life in welded structures. It includes, spot heating and Linde’s method [6]. In local heating method, the structure is heated locally so as to produce local yielding resulting in compressive thermal stresses. On the other hand, Linde’s method is a low-temperature stress relaxation method [6]. Jang et al. demonstrated retardation of a through thickness fatigue crack by local heating [6]. There are also reports of rehabilitation of welds by hammer peening [7] and ultrasonic peening [8]. In recent years, laser has emerged as an effective tool for enhancing fatigue life [9], [10], [11], [12], [13], [14]. Laser surface treatments which affect fatigue life rely on (i) generation of compressive residual stresses by phase transformation or by shock peening and (ii) tensile stresses from non-elastic thermal deformation [15], [16]. Altus et al. used solid-state laser surface treatment to heal initial fatigue damage in Ti–6Al–4 V alloy [17]. Authors attributed healing to yielding and concluded that the laser treatment could be used multiple times as a practical repair method to erase fatigue damage up to the appearance of macro crack. Yee et al. reported an innovative solid-state CO2 laser surface treatment to retard fatigue crack growth in 2024-T3 aluminum alloy [18]. Fatigue crack retardation was attributed to introduction of sufficiently high tensile residual stress at a region in front of the crack, which served to reduce magnitude of maximum shear stress. In the last decade, laser shock peening has rapidly emerged as an effective surface treatment for enhanced fatigue life of engineering components [19], [20], [21], [22]. The process exploits laser-generated shock waves to introduce high level of surface compressive stresses into the substrate. It involves irradiation of the substrate with high-energy short laser pulses causing instantaneous vaporization of the surface layer into high-temperature high-pressure plasma. Rapid expansion of the resultant plasma from the surface generates a high-pressure shock wave, which propagates into the substrate. When peak pressure of the shock wave exceeds dynamic yield stress of the substrate (Huguniot elastic limit), the metal is plastically deformed, thereby generating compressive residual stress on the surface of the substrate [23]. Tran etal [19] have reported that laser shock peening, in spite of being capital intensive with high running cost, is an effective fatigue life extension technology. An extensive survey conducted by Spradlin et al. [20] demonstrated fatigue life extension by laser shock peening (LSP) for a variety of specimen shapes, loadings, and materials. Hatamleh [21] used LSP to achieve significant increase in fatigue properties for failures involving surface-initiated cracks. Tan et al. [22] reported effective suppression of fatigue crack growth in Al alloys with various pre-existing notch configurations by LSP.
A recent study performed on SAE 9260 spring steel in authors’ laboratory demonstrated significant increase in fatigue life over untreated as well as conventionally shot peened specimens [24]. The experimental study was carried to evaluate laser shock peening process as an alternative to existing shot peening practice for enhancing fatigue life of leaf springs. The present experimental study, an extension of the previous study, aims to evaluate laser shock peening process to rejuvenate partly fatigue damaged spring steel specimens.
Section snippets
Experimental details
The approach adopted for the present experimental study involved LSP of partly fatigue tested (about 50% of its expected fatigue life) specimens and comparison of their fatigue life with untreated specimens. The study was performed in 2 parts, viz. (i) characterization of fatigue life of the specimens, (ii) fatigue testing of specimens up to 50% of their expected life, followed by LSP of these partly fatigue-tested specimens and their fatigue testing. Similar approach has also been adopted by
Base metal
The base metal specimens exhibited tempered martensite microstructure (Fig. 2) with Vickers micro-hardness (HV) value in the range of 440–480 HV (load = 1.96 N). The heat treated specimens in the as-received condition, carried 50–100 μm thick decarburized surface layer, as shown in Fig. 2. In the decarburized layer, the material suffered a drop in micro-hardness to 250–330 HV (load = 1.96 N), as shown in Fig. 3. In order to avoid adverse effect of soft decarburized layer on the fatigue life of heat
Conclusions
The study demonstrates effectiveness of laser shock peening as a practical surface treatment for extending life of partly fatigue-damaged in-service spring steel components. Significant extension in the fatigue life of spring steel specimens was brought about by introduction of surface compressive stresses with associated surface hardening effects without adversely affecting their original surface finish. Commercially available PVC-base insulation tape has been found to be a superior
Acknowledgements
Authors are thankful to Mr. Ram Nihal Ram, Mr. Raju Vishwakarma, Mr. K.S. Deohare and Mr. Mahendra Babu for their technical support during various stages of the investigation. They wish to thank Mr. B. Tirumala Rao and Dr. Mukesh Joshi for arranging surface roughness measurements.
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